1. Field of the Invention
The present invention relates to a multivalued information recording method and apparatus and a recording medium for recording multivalued information onto an optical recording medium whose characteristic varies according to applied energy.
2. Description of the Related Art
In recent years, optical disks have become of higher density. For optical disks capable of recording and reproduction of information such as CD-Rs and denser DVD-RAMs which are presently in common use, binary recording is generally used.
That is, laser power corresponding to one-bit digital information, namely “0” and “1”, is applied onto the recording surface of the optical disk, the recording film formed on the recording surface, for example, a so-called phase-change film made of a GeTeSb compound is crystallized or made amorphous, and by the change in reflectance caused thereby, information is recorded.
In other words, binary, that is, one-bit information can be recorded in one information unit, that is, a partition unit on the medium (for example, the length on the recording surface corresponding to one clock) in recording information.
On the other hand, a multivalued information recording method has been proposed in recent years. For example, as shown in Japanese Laid-open Patent Application No. Hei 6-309720, laser power is switched among three or more values, and marks of sizes corresponding to the values are formed on the recording medium. For example, by forming marks of different sizes using laser power of four values corresponding to two-bit information “0(00)”, “1(01)”, “2(10)” and “3(11)”, twice as much information per information unit can be recorded.
However, merely recording marks of sizes corresponding to the four values as described above sometimes causes reproduction errors because of interference between symbols when the marks are reproduced.
That is, in the case that the four-valued information is reproduced, for example, judgement for two-bit information “0”, “1”, “2” and “3” is based on the reproduction signal amplitude that varies according to the sizes of the mark. When the reproduction signal amplitude varies due to interference between symbols from the adjacent information marks, the information is wrongly judged as “3” where it should be judged as “2”.
The problem of interference between symbols is not peculiar to the multivalued recording but yields also in the conventional binary recording. However, while the binary recording has a margin corresponding to the amplitude difference between reproduction signals caused by the presence or absence of the mark, the four-valued recording has a smaller margin corresponding only to a quarter of the amplitude difference, so that the problem of interference between symbols is more serious in the four-valued recording.